mod_rewrite issue

I have a BuddyPress installation working pretty well at http://pmasocial.com. I am running WPMU version 2.8.6 and have configured it to use subdirectories. The installation is in the root of the site’s web directory. I am running BuddyPress 1.1.3, and both WPMU & BP were fresh installations. I am using a custom theme but have not modified the core files in any way. I have custom functions. I also have bbPress installed but have no idea how to determine which version.

The one problem (I’m aware of) right now is that there is evidently an issue with either the .htaccess file or mod_rewrite itself, which causes a redirect to the 404 page whenever I attempt to delete an item from my activity feed at this URL: